In video display apparatuses, there may occur a phenomenon in which an image remains on the screen as an afterimage.
In CRT displays and plasma displays, the deterioration rate of the fluorescent substance attached to the display is different depending on the brightness of the image. Therefore, if an identical image is displayed for a long time, some parts degrade fast and other parts degrade slowly. The part that degrades fast and the part that degrades slowly produce a difference in brightness, and this difference in brightness generates an afterimage on the screen. This afterimage is called burn-in.
On the other hand, in liquid crystal displays, differing from the burn-in, there occurs a phenomenon in which an afterimage continues to remain on the screen for a long time if an identical image has displayed for a long period.
Since, in a liquid crystal display, video signals are applied to the liquid crystal in the form of alternating current, normally, a voltage having a waveform that is symmetrical on both the positive and negative sides is applied.
However, the potential of a pixel electrode may deviate from the potential of the signal line through which the video signal is supplied to the pixel electrode, due to the parasitic capacitance occurring between the gate electrode and the source electrode of the TFT (Thin Film Transistor) of the pixel and the retention capacitance for retaining the voltage applied to the pixel electrode. This deviation of potential differs depending on the amplitude of the video signal.
If the potential of the pixel electrode deviates from the potential of the signal line, a DC offset occurs in the liquid crystal, so that the voltage applied to the liquid crystal deviates from the waveform that is symmetrical on the negative and positive sides. In this case, it is possible to correct the voltage to be applied to the liquid crystal to that of a waveform substantially symmetrical on the positive and negative sides if a DC (direct current) component is added to the video signal to cancel the DC offset. However, since the capacitance of the liquid crystal varies in accordance with the amplitude of the video signal due to dielectric anisotropy of the liquid crystal, it is impossible to correct the voltage applied to the liquid crystal to that of a perfect symmetric waveform on the positive and negative sides.
If the voltage applied to the liquid crystal deviates from a symmetric waveform, impurities (especially, ions) solved in the liquid crystal adhere to one of the electrodes for the liquid crystal so as to generate an electric field inside the liquid crystal.
If an unvaried image is displayed for long, the deviation of the voltage applied to the liquid crystal from a symmetrical waveform is retained, so that the amount of impurities adhering to the electrode for the liquid crystal increases, resulting in generation of a stronger electric field inside the liquid crystal. This electric field will produce an afterimage.
In this way, in video display apparatuses, if an identical image is displayed for long, the phenomenon in which an afterimage remains on the screen for a long time will occur although the generation mechanism is different between the CRT display and plasma display, and the liquid crystal display.
As technologies for reducing generation of afterimages in video display apparatuses, a video signal processing circuit is disclosed in Patent Document 1 and a liquid crystal device is disclosed in Patent Document 2.
In these inventions, the video signal is integrated so that an afterimage signal that represents the video image remaining on the screen as an afterimage is acquired. Then this afterimage signal is subtracted from the video signal.
Since this enables implementation of afterimage correction to the video image so as to cancel out the afterimage, it is possible to reduce afterimages arising in the video display apparatus.    Patent Document 1: JPH02-092174A    Patent Document 2: JP2003-234980A